The ubiquitous signaling enzyme family, Protein Kinase C, is thought to play a central role in regulation of both contractility and growth in the mammalian heart. We have recently demonstrated by immunofluorescence that one isoform of Protein Kinase C translocates to the Z-line region of cardiac myofibrils in response to specific intracellular lipid signals (Huang et al., J. Cell Sci. in press). We have hypothesized that translocation of this e-isoform underlies the enhancement of cardiac contractility seen with lipid mobilizing agonists. We would now like to identify myofibrillar structures by EM that are in close proximity to this active e-Protein Kinase C, to guide our search for candidate anchoring sites and protein substrates. The location of active Protein Kinase C in relation to the Z-hne, t-tubule membranes and thick and thin filaments are of particular interest.